It will be appreciated by those having ordinary skill in the art that conventional aqueous dyeing processes for textile materials, particularly hydrophobic textile materials, generally provide for effective dyeing, but possess many economic and environmental drawbacks. Particularly, aqueous dyebaths that include organic dyes and co-solvents must be disposed of according to arduous environmental standards. Additionally, heat must be applied to the process to dry the textile material after dyeing in an aqueous bath. Compliance with environmental regulations and process heating requirements thus drive up the costs of aqueous textile dyeing to both industry and the consuming public alike. Accordingly, there is a substantial need in the art for an alternative dyeing process wherein such problems are avoided.
One alternative to aqueous dyeing that has been proposed in the art is the dyeing of textile materials, including hydrophobic textile materials like polyester, in a supercritical fluid. Particularly, textile dyeing methods using supercritical fluid carbon dioxide (SCF-CO.sub.2) have been explored.
However, those in the art who have attempted to dye textile materials, including hydrophobic textile materials, in SCF-CO.sub.2 have encountered a variety of problems. These problems include, but are not limited to, "crocking" (i.e. tendency of the dye to smudge when the dyed article is touched) of the dye on the dyed textile article; unwanted deposition of the dye onto the article and/or onto the dyeing apparatus during process termination; difficulty in characterizing solubility of the dyes in SCF-CO.sub.2 ; difficulty introducing the dyes into the SCF-CO.sub.2 flow; and difficulty in preparing the dyes for introduction into the dyeing process. These problems are exacerbated when attempts to extrapolate from a laboratory process to a plant-suitable process are made.
PCT Publication No. WO 97/13915, published Apr. 17, 1997, designating Eggers et al. as inventors (assigned to Amman and Sohne GmbH and Co.) discloses a system for introducing dye into a CO.sub.2 dyeing process which comprises a bypass flow system associated with the main circulation system that includes a color preparing vessel. The bypass is opened, after a certain temperature and pressure are reached, so that SCF-CO.sub.2 flows through the color preparing vessel and dissolves the previously loaded dye(s). The SCF-CO.sub.2 -containing dissolved dye flows from the bypass back into the main circulation system where it joins the bulk of the SCF-CO.sub.2 flow that is used to accomplish dyeing.
PCT Publication No. WO 97/14843, published Apr. 24, 1997, designating Eggers et al. as inventors (assigned to Amman and Sohne GmbH and Co.) discloses a method for dyeing a textile substrate in at least one supercritical fluid, wherein the textile substrate is preferably a bobbin and the fluid is preferably SCF-CO.sub.2. The disclosed invention attempts to prevent color spots from forming on the textile substrate during dyeing and is directed to ways of incorporating the dye material into the supercritical fluid using the basic bypass system as described above in PCT WO 97/13915.
The method involves the use of at least one dye which is contacted with the supercritical fluid as a dye bed, dye melt, dye solution, and/or dye dispersion before and/or during actual dyeing in an attempt to form a stable solution of dye in the supercritical fluid. A stated goal is avoiding the formation of dye agglomerates having a particle size of more than 30 microns, preferably more than 15 microns, in the solution.
This invention attempts to accomplish these aims through a variety of embodiments. In one embodiment, the dye bed is provided with inert particles, in particularly glass and/or steel balls, to prevent agglomeration. Alternatively, the dye bed itself can consist of inert particles coated with the dye. SCF-CO.sub.2 is then passed through the dye bed to incorporate the dye within the SCF-CO.sub.2.
However, there are a number of significant drawbacks to this embodiment of the dye introduction method disclosed by Eggers et al. PCT Publication No. WO 97/14843. For example, use of a fixed or fluidized bed to introduce dye into the dyeing system can be hindered if appropriate flow conditions are not present. The dye particles must be at all times in intimate and vigorous contact with the supercritical fluid for effective dissolution. If this is not the case, the dissolution rate will be low and will likely not be complete by the end of the dyeing cycle.
Moreover, promotion of a high convective mass transfer coefficient (i.e., intimate and vigorous mixing) can result in substantial pressure losses through the dye-add vessel. Because of their relatively low viscosity values, supercritical fluids are easily diverted to areas of lower resistance, which can lead to mechanical problems such as channeling and stagnation. Channeling refers to the development of a fluid path, or channel, through a particulate bed that circumvents uniform flow throughout the bed; i.e., a stream of fluid develops through the bed such that the flow in the region where the stream exists is greater than the flow of fluid in the rest of the bed. In this case, the particles not in the channel are not properly contacted by the fluid. These conditions, in turn, result in dye particles not being contacted in a manner that will allow substantially complete dissolution.
Insuring the proper flow conditions when using fluidized dye beds, fixed dye beds, or dye bed holding devices requires very careful and complex design of the internals of the dye-add vessel in order to assure good mixing and to avoid mechanical flow problems without excessive pressure drop. Indeed, it is likely that dye bed holding devices that are chambered to force uniform flow of fluid through the bed, such as those proposed for use in dye introduction by Eggers et al., PCT Publication No. WO 97/14843, also suffer very high pressure losses.
Another drawback arises when the fluidized and fixed dye bed is installed in the system in a bypass loop. Since the dye dissolution process is rate limiting, this arrangement couples the dyeing process to the dye dissolution process, which is generally undesirable. In contrast, the dye should be introduced at a rate consistent with dyeing the textile material as rapidly as possible but also in a level manner.
An alternative embodiment of the dye injection method disclosed by Eggers et al. PCT Publication No. WO 97/14843 involves injection of the dye as a melt incorporated in an inert gas, preferably nitrogen or carbon dioxide (with property of being inert for these two gases being a function of the process conditions). It has been observed by the present applicants that melting of disperse dyes can lead to decreased solubility in SCF-CO.sub.2. This circumstance indicates that the applicability of this embodiment of the disclosed dye injection method is limited.
Yet another embodiment of the dye introduction method disclosed by Eggers et al. PCT Publication NO. WO 97/14843 involves delivery of the dye into the supercritical fluid flow as a solution or suspension. When a solution is being injected and water-soluble dyes are being used, the recommended injection solvent is water. For water-insoluble dyes, a variety of common nontoxic injection solvents are suggested, with acetone, which readily dissolves disperse dyes, being foremost. The water-insoluble dyes are injected as a solution or suspension in the chosen solvent. In the case that a suitable nontoxic solvent cannot be found or the required amount of solvent is so great that it adversely affects the dyeing process, injection of a dispersion, preferably an aqueous dispersion, is recommended.
This embodiment of the method disclosed by Eggers et al. PCT Publication No. WO 97/14843 also suffers from several drawbacks. Firstly, water is an anti-solvent in SCF-CO.sub.2 when used with disperse dyes. Thus, for SCF-CO.sub.2, the presence of water results in a significantly impaired dyeing process to the extent that it is questionable whether dyeing could be accomplished at all. At best, the action of water in the SCF-CO.sub.2 would cause the dye to reside in the dyeing process as dispersion. In the worst case, the dye would exist as an unstable suspension with unsuitable properties for dyeing. Secondly, in the case that a suitable SCF-CO.sub.2 /water/dye dispersion was obtained, the SCF-CO.sub.2 dyeing process would be similar to the conventional aqueous process, the replacement of which is a desired goal in the art.
Poulakis et al., Chemiefasern/Textilindustrie, Vol. 43-93, February 1991, pages 142-147 discuss the phase dynamics of supercritical carbon dioxide. An experimental section describing an apparatus and method for dyeing polyester in supercritical carbon dioxide in a laboratory setting is also presented. Thus, this reference only generally describes the dyeing of polyester with supercritical carbon dioxide in the laboratory setting and is therefore believed to be limited in practical application.
U.S. Pat. No. 5,199,956 issued to Schienker et al. on Apr. 6, 1993 describes a process for dyeing hydrophobic textile material with disperse dyes by heating the disperse dyes and textile material in SCF-CO.sub.2 with an azo dye having a variety of chemical structures. The patent thus attempts to provide an improved SCF-CO.sub.2 dyeing process by providing a variety of dyes for use in such a process.
U.S. Pat. No. 5,250,078 issued to Saus et al. on Oct. 5, 1993 describes a process for dyeing hydrophobic textile material with disperse dyes by heating the disperse dyes and textile material in SCF-CO.sub.2 under a pressure of 73 to 400 bar at a temperature in the range from 80.degree. C. to 300.degree. C. Then the pressure and temperature are lowered to below the critical pressure and the critical temperature, wherein the pressure reduction is carried out in a plurality of steps.
U.S. Pat. No. 5,578,088 issued to Schrell et al. on Nov. 26, 1996 describes a process for dyeing cellulose fibers or a mixture of cellulose and polyester fibers, wherein the fiber material is first modified by reacting the fibers with one or more compounds containing amino groups, with a fiber-reactive disperse dyestuff in SCF-CO.sub.2 at a temperature of 70-210.degree. C. and a CO.sub.2 pressure of 30-400 bar. Specific examples of the compounds containing amino groups are also disclosed. Thus, this patent attempts to provide level and deep dyeings by chemically altering the fibers prior to dyeing in SCF-CO.sub.2.
U.S. Pat. No. 5,298,032 issued to Schlenker et al. on Mar. 29, 1994 describes a process for dyeing cellulosic textile material, wherein the textile material is pretreated with an auxiliary that promotes dye uptake subsequent to dyeing, under pressure and at a temperature of at least 90.degree. C. with a disperse dye from SCF-CO.sub.2. The auxiliary is described as being preferably polyethylene glycol. Thus, this patent attempts to provide improved SCF-CO.sub.2 dyeing by pretreating the material to be dyed.
Despite extensive research into SCF-CO.sub.2 textile dyeing processes, there has been no disclosure of a suitable method for introducing dyes or other textile treatment materials into such processes. An environmentally and economically sound method for introducing dyes or other textile treatment materials would be particularly desirable in the plant-scale application of a SCF-CO.sub.2 textile dyeing process. Therefore, the development of such a method meets a long-felt and significant need in the art.